Outline

Background: A multilayer ionization chamber (MLIC) has been developed to measure and verify proton beam Bragg peak curves with variable modulation of the beam energy for the second generation of the PSI spot scanning proton therapy facility, Gantry 2. The detector is designed especially for the pencil beam scanning technique.

Material and methods: The MLIC detector consists of 128 parallel plate ionization chambers stacked in series. One millimeter thick aluminum plates are used as beam energy absorbers as well as chamber cathodes. The applied voltage on the cathodes is -200 volts. Mylar foils (100 μm) coated with 0.1 μm copper are glued on the aluminum plates. The ionization current is collected on the coated copper layer (the anode) with a signal collection area of 10 cm x 10 cm. Each ionization chamber has an air spacing of 1mm. The total water equivalent depth of the stacked array is about 290 mm with a step of 2.26 mm. The electronic read-out is based on TERA06 chips (Developed by University and INFN Torino and Terapia con Radiazioni Adroniche, Italy). The output values of the 128 channels are measured simultaneously. The charge-to-count conversion factor is adjusted as -400fC per count.

Results: We have used the MLIC in proton beam measurements. The measured Bragg peak curves agree very well with our reference data measured in a water phantom. According to our preliminary experimental studies with an Alderson phantom, the MLIC can be utilized as a low dose range probe for assessing the range accuracy in vivo.

Conclusion: Our results proved that the MLIC fulfills the technical requirements of our scanning proton therapy. It will be a time saving tool for routine dosimetry and quality assurance measurements.